Then turn on the little graph icon beside the = icon (the icons that appear below the property value when you add an expression) and switch to the graph editor to see the results of the expression. The = icon is the expression on/off button, the graph icon needs to be on for the post-expression result to appear in the graph editor.

The first value in a wiggle expression controls frequency (wiggles per second) and the second controls the amount. So this example will wiggle the value once every second by up to 100%, centered around the original pre-expression value.

If the original value was 100%, you'll find the resulting values stay at 100% for quite a bit of the time, as the values are clamped at the opacity's max value of 100%.

You can also try something like this which will ensure the resulting value is 100% for even longer:wiggle(1,100)+50;

And then just play around with the values (checking the result in the graph editor) to see if you can get a result you like.